As a surgeon or an assistant manipulates an endoscope with an attached camera, the camera faithfully relates what it sees, with its own upright axis displayed as the upright axis of the image on the display. This often results in rotation of the viewed image. As the image rotates, the surgeon loses track of what is actually up and down inside the endoscopic cavity. This disorientation is one of endoscopy's greatest enemies and has lead to severe mistakes such as the snipping of optical nerves which during a procedure were believed to be a different part of the anatomy. In open procedures, the surgeon can see the anatomy directly and therefore does not have a disorientation problem. However, during an endoscopic procedure the surgeon's viewpoint is different from the viewpoint of the endoscope, and the surgeon must continuously try to correlate his own mental picture of the anatomy with the endoscopic picture on the display. In doing this, the need to know what is up and down inside the endoscopic cavity is so strong that it has become common for surgeons to observe the flow direction of fluid droplets on the endoscope cover window or search for pooling blood in order to get a sense of direction inside the cavity. Aside from being important for distinguishing anatomical features which may look similar, knowing the up direction helps in understanding the endoscope's position relative to the surrounding anatomy. Ideally, the surgeon would be able to relate to the endoscopic cavity as if his own eyes were actually inside the cavity.
Attempted solutions to this problem have been proposed in U.S. Pat. No. 5,307,804 to Bonnet (1994), U.S. Pat. No. 5,899,851 to Koninckx (1999), U.S. Pat. No. 6,097,423 to Mattsson-Boze, et al. (2000), U.S. Pat. No. 6,471,637 to Green, et al. (2002), U.S. patent application Ser. No. 10/093,650 by Chatenever, et al. (2002), and U.S. patent application Ser. Nos. 10/829,767 and 60/560,172 by Schara et al. (2004), which are incorporated herein by reference in their entireties. The objects of these inventions are to provide schemes which can maintain the proper upright gravity-leveled orientation of the endoscopic image regardless of how the endoscope is being manipulated.
None of these solutions address the problem of so-called viewing singularities (poles). In a singular viewing configuration there is no unique upright image orientation. This occurs when the viewing direction (described as a view vector) is parallel to the direction of gravity. Although a mathematical discontinuity exists only at a singularity itself, the effect of the singularity is nearly everywhere and decreases as one moves away from it.
A viewing singularity is similar to standing on the North Pole and having to define which direction is south. In gravity-leveled endoscopic systems singularities cause the endoscopic image to suddenly flip or spin rapidly. This is obviously confusing and annoying to the user. Until now, it has not been clear how one should deal with situations where there is no defined up or down in the endoscopic image.
Thus, it is an object of this invention to provide a method for dealing with singularities in gravity-leveled endoscopic imaging systems such that the endoscopic image does not unexpectedly flip or spin during the endoscopic viewing process. It is an additional object of this invention to be applicable to any axial, oblique, side, or retro viewing endoscope as well as any endoscope with a variable direction of view.